1. Field of the Invention
This invention relates to an arrangement for direct transformation of electric power from one DC-(=direct current) voltage to another DC-voltage.
2. Description of the Prior Art
In power transmission DC-voltage is used to transmit high electric power from production centers to consumption centers. Since the power is generated and distributed with AC networks (1,2) it is necessary to transform the AC-voltage to a DC-voltage (U.sub.d in FIG. 1) by means of a rectifier (3) and on the other end re-transform the DC-voltage to an AC-voltage by means of an inverter (4). These convertors are composed of converter transformers (5, 6) and valves (7) which are connected into valve bridges (8, 9). The rectifier and the inverter as well as the valve bridges are known and described in reference 1, chapter 2 and 3.
The rectifier and the invertor can be provided with filters on the AC-voltage side (10, 11) as well as on the DC-voltage side (12, 13). These filters as well as the smoothing reactor (14, 15) on the DC-voltage side are provided in order to filter harmonics in current and voltage, which are generated as a consequence of the transformation from AC- to DC-voltage and vice versa. Each rectifier or inverter consequently needs a lot of equipment which also generates a lot of losses. This has strongly restricted the utilization of high voltage direct current as a means of transmitting electric power.
With technology known today relating transformation of electric power from one high voltage DC-voltage level to another high voltage DC-voltage level, the power is converted to an AC-voltage by means of an alternator and then converted to the other DC-voltage level by means of a rectifier. Another known arrangement relates to a series connection of a couple of converters for increasing or decreasing of the DC-voltage level in proportion to the power supplied to or withdrawn from the AC-voltage network (compare reference 2).
Known arrangements of DC/DC transformation for low voltage application (se e.g. chapter 7 in reference 3) are not suitable for power transmission and high voltage equipment, due to the high requirement for low noise interference, low losses and high insulation levels, and the high leakage inductances in the transformers related to the high voltage levels.
The known rectifier (3) and inverter (4) are drawn in FIG. 1. In the figure a 12 pulse configuration is illustrated with star- and delta-connected converter transformers, which is the most common configuration today. This known configuration and corresponding firing sequence is described in chapter 2.9 of reference 1. In the 12-pulse configuration the firing varies cyclically from one valve to another in each 12-pulse group (8, 9). The two series connected 6-pulse groups in each rectifier and the inverter are phase shifted 30.degree. since transformer valve windings in the upper group are star connected (16, 18) and in the lower group are delta connected (17, 19). Due to restrictions in maximum power handling capacity of each transformer unit the transformer windings may be divided in one, two, three or six units. In each of these units there must be at least one AC-winding (20, 21) with the same phase shift as the valve windings in the respective transformer unit. The greatest quantity of transformer units and the lowest power handling capacity per unit is achieved if only one valve winding and corresponding AC winding is placed in one and the same transformer unit.
Since rectifying and inversion with today's power technology is performed with line commutated valves, the firing and extinction is achieved only with certain firing angle, .alpha., and extinction angle, .gamma., respectively. Commutation from the valve winding of one phase, to a valve winding of another phase will only be achieved with a certain overlap angle, u due to the transformer leakage inductance. Due to these a certain phase shift between the voltage and the current is created during the rectification and the inversion processes. This results in a deficit of reactive power as described in reference 2. In order to compensate for this it has become useful to provide the convertors not only with ac-filters (10, 11) but also with shunt capacitor banks (22, 23) for generation of reactive power. The DC-current control is an essential function of the known DC-voltage transmission. The line direct current (I.sub.d in FIG. 1) in the known DC voltage transmission is controlled by the DC voltages in the converter stations through the formula: ##EQU1## I.sub.d =Line DC-current U.sub.d.sup.R =DC-voltage in rectifier
U.sub.d.sup.I =DC-voltage in inverter PA1 R=Line resistance PA1 T=Time of a cycle PA1 F=Firing signal PA1 E=Extinction signal ##EQU2## U.sub.A1, U.sub.B1, U.sub.C1 =Phase voltages in inverter valve windings (p.u.) PA1 U.sub.A2, U.sub.B2, U.sub.C2 =Phase voltages in rectifier valve windings (p.u.) PA1 U.sub.A, U.sub.B, U.sub.C =Phase currents (p.u.) PA1 N.sub.1 =number of turns in the valve winding of the inverter (43) and PA1 N.sub.2 =number of turns in the valve winding of the rectifier (45). The inverter side DC line current, I.sub.d1, is determined by the average value of the bridge current EQU I.sub.d1 =I.sub.b1 /(1-3u.sub.T /T). PA1 The rectifier side DC line voltage, U.sub.d2, is determined, by the average value of the bridge voltage EQU U.sub.d2 =U.sub.B2 (1-3u.sub.T /T)).
The DC-voltages are controlled by the firing and extinction angles and the tap changers in the way described in chapter 7 of reference 4.